This invention relates to an electric circuit with limit-switches for limiting rotatable range of an electric drive unit for controlling the degree of opening of an automotive throttle valve.
FIGS. 1 and 2 illustrate an electric drive unit disclosed in Japanese Patent Application No. 62-203766 for example. FIG. 1 is a sectional view of the unit, and FIG. 2 is a view showing the interior of the electric drive unit shown in FIG. 1 with its protective cover removed. In these figures, 26 is a mounting plate to which the electric drive unit is mounted. 25 is a protective cover for protecting the electric drive unit. 16 is a rotary lever constituting an output portion of the electric drive unit. This is a basically circular member having a receiving groove at its periphery. 18 is an external load driving wire, one end thereof being received in the groove of the rotary lever 16 to be connected thereto and the other end thereof extending to a throttle valve (not shown). 10 is an output shaft to which the rotary lever 16 is secured. 7, 6, 5, 4, 3 constitutes a speed reducing gear train connected to the output shaft 10 through a clutch 11. 1 is a motor connected to the speed reducing gear train 7.about.3. This motor 1 drives the above-mentioned rotary lever 16.
Referring to FIG. 1, the other end of the output shaft 10 has connected thereto a limit lever 19. The limit lever 19 abuts against a first and a second span-action limit switches 21a and 21b (FIG. 4) at about 180.degree. rotational position of the output shaft 10 to prevent a further rotation of the rotary lever 16. Such the electric drive unit controls the travel speed of an automobile in place of controlling of the travel speed of an automobile by pressing an accelerator pedal by an operator. That is, the motor 1 rotates in a forward or a reverse direction according to a command from a control unit (not shown). The limit switch lever 19 as well as the first and the second limit switches 21a and 21b are for limiting the above rotation range.
FIG. 3 illustrates a conventional electric circuit in which the above limit switches 21a and 21b are incorporated. In the figure, A and B are input terminals of the motor 1. "a" is an electric current in the direction rotating the motor 1 for pulling the external load driving wire 18 (in the direction opening the throttle valve), and b is an electric current opposite to the above current. D is a normally-closed contact of the first limit switch 21a, and E is a normally-open contact. G is a normally-closed contact of the second limit switch 21b and F is a normally open contact. 30A and 30B are diodes, and C is a common connection point.
The operation will now be described. The motor 1 rotates in accordance with the direction of the current flowing through the terminals A and B as long as the limit lever 19 engages none of the first and the second limit switches 21a and 21b. This is because the diodes 30A and 30B are connected in the opposite directions at the junction C, and the limit switches 21a and 21b are closed at their normally closed contacts D and G. However, when the motor 1 continues to rotate and the limit lever 19 engages the second limit switch 21b to move its movable contact from the normally-closed contact G shown in a solid line to the normally-open contact F shown in a dotted line, the circuit from the motor 1 to the terminal B is disconnected and at the same time a shorting circuit including the diode 30A allowing the current to flow into the motor 1 in the opposite direction. The motor 1 is disconnected from the driving current a and at the same time instantaneously stops due to the regeneration braking. Thereafter, the motor 1 is non-responsive to the current a from a control unit (not shown). This is because no current is supplied to the terminal B from the motor 1 through the normally-open contact F of the second limit switch 21b. However, the current b in the opposite direction flows because its depends on the nature of the conduction direction of the diode 30B. Therefore, when the current b is supplied from the control unit, the motor 1 is rotated in the direction in which the limit lever 19 moves away from the second limit switch 21b, so that the movable contact of the second limit switch 21b returns to the position shown by the solid line i.e., normally-closed contact G. Thus, the current flows from the motor 1 to the terminal B through this normally closed contact G so that the motor 1 continues to rotate.
As the motor 1 continues to rotate and the limit lever 19 engages the first limit switch 21a, the motor 1 instantaneously stops at that position as described previously, becoming non-responsive to the current b from the control unit. However, the motor 1 responses to the current a in the opposite direction.
Thus, the electric drive unit is limited as to its rotational angle independently of the control unit (not shown).
With the conventional electric circuit of the abovedescribed construction, when the limit lever 19 stops after it causes the operating levers 22a and 22b of the snap-action limit-switches 21a and 22b to be located at intermediate positions, and the movable contact of the limit switches 21a and 21b are positioned between the normally-closed contacts D and G and the normally-open contacts E and F so that they are not in contact with either of the contacts, then the electric drive unit becomes completely inoperative (in which the motor 1 does not respond to the current in either direction). For example, if the movable contact of the second limit switch 21b does not engage the normally-closed contact G as well as the normally-open contact F, no current in what ever direction flows into the motor 1. This means that, when the motor 1 is rotated by the current a so as to fully open the throttle valve, and when the movable contact of the second limit switch 21b accidentally stops at an intermediate or a neutral position between the normally-closed contact G and the normally-open contact F, then the throttle valve is held at a fully open state, resulting in a full speed runaway of the automobile under the uncontrolled state.
The probability that the phenomenon of the movable contact of the snap-action limit switch being held at a neutral position as above described takes place is extremely low. For example, the above phenomenon may take place only once in a single automobile out of 10,000 automobiles used for ten years. However, while it cannot be determined as to when, where and in which automobile the above uncontrolled runaway phenomenon occurs, a make of automobile including some in which the above phenomenon occurs are inevitably included from the stand point of statistics is a defective automobile.
On the other hand, if the movable contact of the first limit switch 21a stops at the neutral position between the normally-open contact E and the normally-closed contact D, then the electric drive unit fails to operate in the direction toward accelerating the automobile, which is also a defect although not runaway.